Arguments

the object to be written, preferably a matrix or data frame.
If not, it is attempted to coerce x to a data frame.

file

either a character string naming a file or a connection
open for writing. "" indicates output to the console.

append

logical. Only relevant if file is a character
string. If TRUE, the output is appended to the
file. If FALSE, any existing file of the name is destroyed.

quote

a logical value (TRUE or FALSE) or a
numeric vector. If TRUE, any character or factor columns
will be surrounded by double quotes. If a numeric vector, its
elements are taken as the indices of columns to quote. In both
cases, row and column names are quoted if they are written. If
FALSE, nothing is quoted.

sep

the field separator string. Values within each row of
x are separated by this string.

eol

the character(s) to print at the end of each line (row).
For example, eol = "\r\n" will produce Windows' line endings on
a Unix-alike OS, and eol = "\r" will produce files as expected by
Excel:mac 2004.

na

the string to use for missing values in the data.

dec

the string to use for decimal points in numeric or complex
columns: must be a single character.

row.names

either a logical value indicating whether the row
names of x are to be written along with x, or a
character vector of row names to be written.

col.names

either a logical value indicating whether the column
names of x are to be written along with x, or a
character vector of column names to be written. See the section on
‘CSV files’ for the meaning of col.names = NA.

qmethod

a character string specifying how to deal with embedded
double quote characters when quoting strings. Must be one of
"escape" (default for write.table), in which case the
quote character is escaped in C style by a backslash, or
"double" (default for write.csv and
write.csv2), in which case it is doubled. You can specify
just the initial letter.

fileEncoding

character string: if non-empty declares the
encoding to be used on a file (not a connection) so the character data can
be re-encoded as they are written. See file.

Details

If the table has no columns the rownames will be written only if
row.names = TRUE, and vice versa.

Real and complex numbers are written to the maximal possible precision.

If a data frame has matrix-like columns these will be converted to
multiple columns in the result (viaas.matrix)
and so a character col.names or a numeric quote should
refer to the columns in the result, not the input. Such matrix-like
columns are unquoted by default.

Any columns in a data frame which are lists or have a class
(e.g., dates) will be converted by the appropriate as.character
method: such columns are unquoted by default. On the other hand,
any class information for a matrix is discarded and non-atomic
(e.g., list) matrices are coerced to character.

Only columns which have been converted to character will be quoted if
specified by quote.

The dec argument only applies to columns that are not subject
to conversion to character because they have a class or are part of a
matrix-like column (or matrix), in particular to columns protected by
I(). Use options("OutDec") to control
such conversions.

In almost all cases the conversion of numeric quantities is governed
by the option "scipen" (see options), but with
the internal equivalent of digits = 15. For finer control, use
format to make a character matrix/data frame, and call
write.table on that.

These functions check for a user interrupt every 1000 lines of output.

If file is a non-open connection, an attempt is made to open it
and then close it after use.

To write a Unix-style file on Windows, use a binary connection
e.g. file = file("filename", "wb").

CSV files

By default there is no column name for a column of row names. If
col.names = NA and row.names = TRUE a blank column name
is added, which is the convention used for CSV files to be read by
spreadsheets. Note that such CSV files can be read in R by

read.csv(file = "<filename>", row.names = 1)

write.csv and write.csv2 provide convenience wrappers
for writing CSV files. They set sep and dec (see
below), qmethod = "double", and col.names to NA
if row.names = TRUE (the default) and to TRUE otherwise.

write.csv uses "." for the decimal point and a comma for
the separator.

write.csv2 uses a comma for the decimal point and a semicolon for
the separator, the Excel convention for CSV files in some Western
European locales.

These wrappers are deliberately inflexible: they are designed to
ensure that the correct conventions are used to write a valid file.
Attempts to change append, col.names, sep,
dec or qmethod are ignored, with a warning.

CSV files do not record an encoding, and this causes problems if they
are not ASCII for many other applications. Windows Excel 2007/10 will
open files (e.g., by the file association mechanism) correctly if they
are ASCII or UTF-16 (use fileEncoding = "UTF-16LE") or perhaps
in the current Windows codepage (e.g., "CP1252"), but the
‘Text Import Wizard’ (from the ‘Data’ tab) allows far
more choice of encodings. Excel:mac 2004/8 can import only
‘Macintosh’ (which seems to mean Mac Roman), ‘Windows’
(perhaps Latin-1) and ‘PC-8’ files. OpenOffice 3.x asks for
the character set when opening the file.

There is an IETF RFC4180 (http://tools.ietf.org/html/rfc4180)
for CSV files, which mandates comma as the separator and CRLF line
endings. write.csv writes compliant files on Windows: use
eol = "\r\n" on other platforms.

Note

write.table can be slow for data frames with large numbers
(hundreds or more) of columns: this is inevitable as each column could
be of a different class and so must be handled separately. If they
are all of the same class, consider using a matrix instead.